Re: [patch 00/21] mutex subsystem, -V14

[Ingo Molnar <mingo@elte.hu>]

* Joel Schopp <jschopp@austin.ibm.com> wrote:

> Took a glance at this on ppc64.  Would it be useful if I contributed 
> an arch specific version like arm has?  We'll either need an arch 
> specific version or have the generic changed.

feel free to implement an assembly mutex fastpath, and it would 
certainly be welcome and useful - but i think you are wrong about SMP 
synchronization:

> Anyway, here is some disassembly of some of the code generated with my 
> comments:
> 
> c00000000049bf9c <.mutex_lock>:
> c00000000049bf9c:       7c 00 06 ac     eieio
> c00000000049bfa0:       7d 20 18 28     lwarx   r9,r0,r3
> c00000000049bfa4:       31 29 ff ff     addic   r9,r9,-1

> The eieio is completly unnecessary, it got picked up from 
> atomic_dec_return (Anton, why is there an eieio at the start of 
> atomic_dec_return in the first place?).

a mutex is like a spinlock, it must prevent loads and stores within the 
critical section from 'leaking outside the critical section' [they must 
not be reordered to before the mutex_lock(), nor to after the 
mutex_unlock()] - hence the barriers added by atomic_dec_return() are 
very much needed.

	Ingo